Cyclical and stochastic thermal variability affects survival and growth in brook trout

Warming, stochastic diel thermal fluctuations affect physiological performance and gill plasticity in an amphibious mangrove fish

Natural temperature variation in many marine ecosystems is stochastic and unpredictable, and climate change models indicate that this thermal irregularity is likely to increase. Temperature acclimation may be more challenging when conditions are highly variable and stochastic, and there is a need for empirical physiological data in these thermal environments. Using the hermaphroditic, amphibious mangrove rivulus (Kryptolebias marmoratus), we hypothesized that compared with regular, warming diel thermal fluctuations, stochastic warm fluctuations would negatively affect physiological performance. To test this, we acclimated fish to: (1) non-stochastic and (2) stochastic thermal fluctuations with a similar thermal load (27-35°C), and (3) a stable/consistent control temperature at the low end of the cycle (27°C). We determined that fecundity was reduced in both cycles, with reproduction ceasing in stochastic thermal environments. Fish acclimated to non-stochastic thermal cycles had growth rates lower than those of control fish. Exposure to warm, fluctuating cycles did not affect emersion temperature, and only regular diel cycles modestly increased critical thermal tolerance. We predicted that warm diel cycling temperatures would increase gill surface area. Notably, fish acclimated to either thermal cycle had a reduced gill surface area and increased intralamellar cell mass when compared with control fish. This decreased gill surface area with warming contrasts with what is observed for exclusively aquatic fish and suggests a preparatory gill response for emersion in these amphibious fish. Collectively, our data reveal the importance of considering stochastic thermal variability when studying the effects of temperature on fishes.

Effect of thermal variation on the cardiac thermal limits of a eurythermal marine teleost (Girella nigricans)

Although most animals live in complex, thermally variable environments, the impact of this variability on specific physiological systems is still unresolved. The ectotherm heart is known to change in both structure and function to ensure appropriate oxygen delivery under different thermal regimes, but the plasticity of the upper thermal limits of the heart under stable or variable thermal acclimation conditions remains unknown. To investigate the role of thermal variability on cardiac acclimation potential, we acclimated a eurythermal fish, opaleye (Girella nigricans), to three static temperature treatments (13, 16, and 19 °C) as well as two oscillating treatments which cycled between maximum and minimum temperatures every 12 h (13–19 °C and 16–22 °C). These temperatures and daily thermal ranges were chosen to mimic the conditions observed in the rocky intertidal environments in Santa Barbara, CA, USA where the fish were collected. We hypothesized that increasing temperature would increase upper thermal limits of the heart, and that variable acclimations would result in broader acute thermal performance curves (TPCs) compared to static acclimations. We measured maximum heart rate during acute warming to determine cardiac thermal performance (i.e., the temperature corresponding to the onset of cardiac arrythmia, the temperature at maximum heart rate, absolute maximum heart rate, and the Arrhenius breakpoint temperature) and construct acute TPCs. Rising static acclimation temperatures increased upper thermal limits but had no impact on peak maximum heart rate. The warmest static temperature did, however, cause a narrowing of the acute TPC. Fish acclimated to variable conditions had the same upper thermal limits compared to fish acclimated to static conditions with the same mean temperature in all metrics of thermal performance. Further, there was no significant broadening of the acute TPC. This study suggests that cardiac plasticity is robust to thermal variation in this eurythermal fish.

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Rising static acclimation temperatures increased cardiac upper thermal limits.

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Warm acclimation caused the acute thermal performance curve to narrow.

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Acclimation to static and variable thermocycles did not impact upper thermal limits.

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Variable acclimation conditions did not broaden the acute thermal performance curve.

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Cardiac plasticity is robust to thermal variation in some eurythermal fishes.

Thermal performance curves for aerobic scope in a tropical fish (Lates calcarifer): flexible in amplitude but not breadth

Aerobic metabolic scope is a popular metric to estimate the capacity for temperature-dependent performance in aquatic animals. Despite this popularity, little is known of the role of temperature acclimation and variability in shaping the breadth and amplitude of the thermal performance curve for aerobic scope. If daily thermal experience can modify the characteristics of the thermal performance curve, interpretations of aerobic scope data from the literature may be misguided. Here, tropical barramundi (Lates calcarifer) were acclimated for ∼4 months to cold (23°C), optimal (29°C) or warm (35°C) conditions, or to a daily temperature cycle between 23 and 35°C (with a mean of 29°C). Measurements of aerobic scope were conducted every 3-4 weeks at three temperatures (23, 29 and 35°C), and growth rates were monitored. Acclimation to constant temperatures caused some changes in aerobic scope at the three measurement temperatures via adjustments in standard and maximum metabolic rates, and growth rates were lower in the 23°C-acclimated group than in all other groups. The metabolic parameters and growth rates of the thermally variable group remained similar to those of the 29°C-acclimated group. Thus, acclimation to a variable temperature regime did not broaden the thermal performance curve for aerobic scope. We propose that thermal performance curves for aerobic scope are more plastic in amplitude than in breadth, and that the metabolic phenotype of at least some fish may be more dependent on the mean daily temperature than on the daily temperature range.

Effects of seawater acclimation at constant and diel cyclic temperatures on growth, osmoregulation and branchial phospholipid fatty acid composition in rainbow trout Oncorhynchus mykiss

The study investigated the effects of seawater acclimation at constant and diel temperatures on the growth, osmoregulation, and branchial phospholipid fatty acid (PLFA) composition in rainbow trout (Oncorhynchus mykiss). The fish (initial weight, 62.28 ± 0.41 g) were reared at a constant 13.0 °C (CT) or with a diel cycle of either 13.0 ± 1.0 °C (VT2) or 13.0 ± 2.0 °C (VT4) for 6 weeks, and subsequently subjected to seawater acclimation. Diel temperature variations (of up to 4 °C) did not affect the growth rate of rainbow trout maintained in freshwater, but alleviated the impairment on the growth after seawater challenge. Under all temperature conditions, rainbow trout were well prepared to seawater acclimation. The diel cyclic temperature resulted in fish with reduced fluctuations in plasma electrolyte levels, branchial Na⁺-K⁺ ATPase activity, and plasma osmolality. In freshwater, the sum of the monounsaturated fatty acids was significantly higher in the VT4 relative to CT and VT2 treatment. Conversely, the sum of polyunsaturated fatty acids was significantly lower in the VT4 fish. After seawater transfer, the branchial PLFA profiles of the fish significantly changed, but those in CT and VT2 did not recover afterwards (the degree of unsaturation was downregulated). The PLFA composition of fish in the VT4 treatment appeared to be steadier under seawater acclimation. This study suggests that a diel cyclic temperature (13.0 ± 2.0 °C) can alleviate the impairment of growth, enhance osmoregulation capability, and improve the stability of the branchial PLFA composition in rainbow trout after seawater acclimation.